Valve piezo electric

ABSTRACT

A fluid valve comprising a valve member ( 6 ) arranged to be movable in a first direction to control fluid flow through an orifice ( 5 ) and electrically actuable means ( 1   a   , 1   b ) for controlling movement of said valve member, characterised in that the electrically actuable means comprises at least one bender element ( 1   a ) disposed in said first direction but bendable in a direction transverse to said first direction, and the valve member is provided with a profiled surface ( 8 ) engaging one end ( 3   a ) of the bender whereby the position of the valve member is controlled.

The use of active materials such as piezo ceramics and electrostrictivesis known in the field of fluid valves. The primary mode of operation forsuch devices is a cantilever construction that bends to cover anduncover a port. This approach is simple, but has the primarydisadvantage of exerting relatively low sealing forces.

High sealing forces can be achieved through the use of soft seals andexpensive ceramic or bender technologies. There is a requirement for aform of valve that is capable of producing high sealing forces from alow cost actuator.

The present invention provides a fluid valve comprising a valve memberarranged to be movable in a first direction to control fluid flowthrough an orifice and electrically actuable means for controllingmovement of said valve member, characterised in that the electricallyactuable means comprises at least one bender element disposed in saidfirst direction but bendable in a direction transverse to said firstdirection, and the valve member is provided with a profiled surfaceengaging one end of the bender whereby the position of the valve memberis controlled.

In order that the present invention be more readily understoodembodiments thereof will now be described by way of example withreference to the accompanying drawings in which:

FIG. 1 shows a valve according t the present invention in an “off”condition;

FIG. 2 shows the valve of FIG. 1 but in the “on” condition;

FIG. 3 shows diagrammatically a modification to the valve shown in FIGS.1 and 2; and

FIG. 4 shows diagrams to explain the construction and operation to afurther embodiment of the valve shown in FIGS. 1 and 2.

According to the preferred embodiment of the present invention, anormally shaped valve orifice is provided having a soft elastomeric sealof a type that is typically used in pneumatic valve applications.Generally, for mains pressure pilot valves the orifice is a smooth hole5 of 0.7 or 1.0 mm diameter, and the seal is a nitrile rubber. The sealis formed by or provided on a sealing member 6 and this member 6 issuitable shaped to move smoothly within a suitable channel or bore 7such that it can slide within the bore but not rotate or rock. The rearportion of the member has a camming surface 8 in the form of a segmentof a cylinder.

For a system wherein the actuator is capable of movement ofapproximately 2 mm the radius of the cylinder is preferably 7.5 mm andthe centre of the cylinder is 5 mm below the back surface. The channelor bore 7 also supports one and preferably two piezo ceramic orelectrostrictive bender devices 1. These can be of any convenientconstruction but are preferably and advantageously planar bimorphs asspecified in EP-A-0265147.

For increased stiffness these planar bimorphs are preferably but notexclusively of the three-prong type and may be a multiplicity of suchbenders to increase the system force. The benders are mounted at theirmounting points 10 such that they have an end deflection of 2 mm whenexcited. It will be seen by simple geometry that an offset of 2 mm fromthe centre line of a 7.5 mm radius circle creates an angle to the centreline of that circle of approximately 15°.

These dimensions can be adjusted to achieve the same effect with anysuitable combinations of displacements and offsets. Two actuators arepositioned symmetrically either side of the centre line to form anincluded angle of 30°. It is a feature of bender type actuators that asthey deform and change position in the Y-plane the X-plane mustnecessarily shorten, as the material is effectively curling.

If the bender actuators were placed parallel to the centre line of thecylinder on the carrier when excited the X dimension would seduce. Tocounteract this effect the actuators are rotated with respect to thecentre line of the cylindrical cam so that their maximum X dimension inthe system is achieved when they have been excited. For a 2 mmdeflection this rotation on a three-prong bimorph is approximately 5°.

From the illustration it will be seen that the assemblies thus fardescribed comprises a V of two actuators which are cupping thecylindrical feature of the rear of the carrier.

Consider now the operation of the system:

As a voltage is applied to the actuators 1 a, 1 b they will deformbringing their tips 3 a, 3 b towards the centre line of the assembly.The force exerted by the devices towards the centre is translated bysimple geometry to a force approximately four times that exerted in thedirection of motion but with one quarter of the movement. Because theangular relationship of the tip to the cam changes as the partprogresses forwards in the manner of atypical over-centre mechanism thefurther towards the centre that the actuators move the less the movementof the seal towards the orifice but the greater the sealing forces. Withthe dimension thus far described of a 2 mm stroke on a 7.5 mm radius camthe total axial motion of the seal with be approximately 0.35 mm. Toobtain clean air-flow from a 1 mm orifice a clearance of 0.25 mm isrequired. The additional 0.1 mm provides compression of the seal and thenecessary clamping force.

FIG. 1 shows the actuators in their rest condition and FIG. 2 shows themin the fully excited condition wherein they are effectively touching atthe centre of the circle. In this state the pressure exerted by theair-flow and any return spring is effectively opposed by a column incompression providing a stiff system. Preferably but not exclusively thecylindrical cam be made substantially cylindrical with a flat portion atthe centre to completely eliminate any sideways thrusts when the systemis fully excited.

Similarly the inherent resilience of the system can be exploited andthis portion can be made a dip to provide a locking action which willrequire the piezo actuator to be pulsed outwards to release as shown inFIG. 3.

The valve thus far described is a single acting valve having less forcein its open condition than in its closed condition. Where it isdesirable to provide a two position valve a second configuration ofactuators performing the same function can be disposed as shown in FIG.4. Actuation of the first set of actuators 1 on camming surface 8displaces a carrier 40 and brings similar camming surfaces 41 intocontact with a second set of actuators 2 which can then be actuated toforce the carrier 40 back to its original position. It will be seen thatthe carrier 40 replaces the member 6 of FIGS. 1, 2 and 3.

There are several kinds of valve which are known to provide efficientswitching. The valve thus far described is typically termed a poppetvalve. The same construction may conveniently form a sliding valvewherein the motion of the frame aligns slits, holes or similar featuresto permit air-flow or to divert it from one channel to another.

It will be seen from the above description and accompanyingillustrations that the fundamental principle can be applied in a numberof different ways and that the mechanism itself need not apply merely tothe control of air, but may be used for the switching of powders,liquids and mechanical systems.

It is particularly advantageous to use a cam of this type in preferenceto a toggle mechanism because the motion and forces available from piezoand electrostrictive actuators tend to be small but of high force. Thelosses that build up within a pivoted toggle system do not permit longlife and repeatable actuation without high levels of engineeringprecision, such as the use of conical jewel-type bearings.

The use of living hinge and flexing member constructions is notpreferred, because such systems tend to have a short life, are stiff andchange over temperature.

To achieve the life cycles required of typical pneumatic valves which is1×10⁹ operations the tribology and management of the mating surfaces iscritical. The surfaces can be made of any suitable material and it isknown that PTFE loaded plastics operating on ceramic or similarmaterials are effective, as are ceramic to ceramic systems.

Further foreseeable improvements include the alteration of the cammingprofile to provide multiple points at which the system may be stopped topermit full flow, restricted flow and complete shut off.

What is claimed is:
 1. A fluid valve comprising a valve member (6)arranged to be movable in a first direction to control fluid flowthrough an orifice (5) and electrically actuable means (1 a, 1 b) forcontrolling movement of said valve member (6), characterised in that theelectrically actuable means (1 a, 1 b) comprises at least one benderelement (1 a) disposed in said first direction but bendable in adirection transverse to said first direction, and the valve member isprovided with a profiled surface (8) engaging one end (3 a) of thebender whereby the position of the valve member is controlled.
 2. Afluid valve according to claim 1, wherein the electronically actuablemeans comprises two benders (1 a, 1 b) arranged to bend in oppositedirections, and the profiled surface comprises oppositely directedcurves.
 3. A fluid valve according to claim 1, wherein the valve member(6) is located in a channel (7) and constrained to reciprocate in onlysaid first direction within the channel (7).
 4. A fluid valve accordingto claim 2, wherein the valve member comprises frame (40), one internalsurface of which is provided with the profiled surface.
 5. A fluid valveaccording to claim 4, wherein the frame two opposite surfaces providedwith profiles of different shapes (8,41) and the electricallycontrollable means comprises two bender arrangements (1,2) arrangedantiparallel to each other, the frame being moveable between first andsecond positions, in the first of which one of the profiles abuts one ofthe bender arrangements and in the other of which the other profileabuts the other bender arrangement.
 6. A fluid valve according to claim1 wherein the profile is shaped to provide an over-centre actionrequiring the bender to be actuated to move between first and secondpositions and back again.
 7. A fluid valve according to claim 1, whereinthe valve member comprises frame (40), one internal surface of which isprovided with the profiled surface.